107BOnline and diagnostic tools
14.10 Monitoring and modifying values in the CPU
S7-1200 Programmable controller
646
System Manual, 11/2011, A5E02486680-05
To monitor the tags, you must have an online connection to the CPU.
The various functions can be selected using the buttons at the top of the watch table.
Enter the tag name to monitor and select a display format from the dropdown selection. With
an online connection to the CPU, clicking the "Monitor" button displays the actual value of
the data point in the "Monitor value" field.
14.10.4.1
Using a trigger when monitoring or modifying PLC tags
Triggering determines at what point in the scan cycle the selected address will be monitored
or modified.
Table 14- 4 Types of triggers
Trigger
Description
Permanent
Continuously collects the data
Permanent: Continuously collects the data at the start of the scan cycle, after
the CPU reads the inputs
At scan cycle start
Once: Collects the data at the start of the scan cycle, after the CPU reads the
inputs
Permanent: Continuously collects the data at the end of the scan cycle, before
the CPU writes the outputs
At scan cycle end
Once: Collects the data once at the end of the scan cycle, before the CPU
writes the outputs
Permanent: Continuously collects data when the CPU transitions to STOP
At transition to
STOP
Once: Collects the data once after the CPU transitions to STOP
For modifying a PLC tag at a given trigger, select either the start or the end of cycle.
●
Modifying an output: The best trigger event for modifying an output is at the end of the
scan cycle, immediately before the CPU writes the outputs.
Monitor the value of the outputs at the beginning of the scan cycle to determine what
value is written to the physical outputs. Also, monitor the outputs before the CPU writes
the values to the physical outputs in order to check program logic and to compare to the
actual I/O behavior.
●
Modifying an input: The best trigger event for modifying an input is at the start of the
cycle, immediately after the CPU reads the inputs and before the user program uses the
input values.
If you are modifying inputs the start of the scan cycle, you should also monitor the value
of the inputs at the end of the scan cycle to ensure that the value of the input at the end
the scan cycle has not changed from the start of the scan cycle. If there is a difference in
the values, your user program may be writing to an input in error.
To diagnose why the CPU might have gone to STOP, use the "Transition to STOP" trigger to
capture the last process values.
